Simplify SVGResourcesCycleSolver::resourceContainsCycles

Modify the child traversal to use pre-order. This allows the case
for |renderer| to be sunk into the loop, reducing the amount of
code somewhat.
This also fixes a bug where the child traversal would not traverse its
the children of node that did not reference any resources.

BUG=351713

Review URL: https://codereview.chromium.org/294393004

git-svn-id: svn://svn.chromium.org/blink/trunk@174862 bbb929c8-8fbe-4397-9dbb-9b2b20218538